1. Technical Field of the Invention
The present invention relates to a material for collecting radionuclides and heavy metals.
The material of the present invention is used in collecting and concentrating radionuclides present in liquid radioactive wastes discharged from nuclear power plants and other facilities using radioisotopes such as hospitals and non-destructive testing laboratories. The material is also used in collecting and concentrating radionuclides and heavy metals present in the seawater, rivers and industrial wastewaters.
2. Prior Art
The seawater and rivers have been proved to contain trace levels of artifically induced radionuclides such as .sup.60 Co, .sup.54 Mn, .sup.90 Sr, .sup.65 Zn and .sup.137 Cs. Efficient and selective collecting and concentration of such radionuclides is essential for ensuring accurate radioassay and precise detection and determination of nuclides by analytical apparatuses. This requirement is vital if the nuclides to be detected or assayed are those of extremely low levels present in samples taken from the environment. Various sparingly inorganic and organic adsorbents are currently used in collecting and concentrating radionuclides present in extremely low levels in aqueous solutions such as those from the seawater and rivers. The selection of the proper adsorbents depends on the chemical properties of the target nuclide. Attempts are being made to perform a rapid, simple and selective collecting of radionuclides by supporting such adsorbents on specific carriers such as silica gel and particulate resins without attaching them by a strong chemical bond.
Radionuclides are also present in radioactive liquid wastes discharged from nuclear power plants and other facilities using radioisotopes such as hospitals and non-destructive testing laboratories. From a safety aspect, it is necessary to reduce the radioactivity level of such liquid wastes by removal of the radionuclides.
There are two conventional methods for separating and removing radionuclides from radioactive liquid wastes: precipitation by coagulation, and ion-exchanging. In the first method, a coagulant is added to the waste to neutralize the electrical charge on the individual particles of radioactive material, which then agglomerate to form larger particles or "flocs" which settle by gravity to come out of the solution. Commonly employed coagulants are Al.sub.2 (SO.sub.4).sub.3 +Ca(OH).sub.2, clay (+polymeric coagulant), FeCl.sub.3 +Na.sub.2 S, and Na.sub.3 PO.sub.4 +Ca(OH).sub.2.
The first method achieves a not so high decontamination factor but is extensively used in treating a large volume of liquid waste having a simple composition. On the other hand the ion-exchange method is very good in respect of the decomposition and permits the use of many types of exchangers such as synthetic cation-exchangers, anion-exchangers, mixedbed ion-exchangers, natural organic exchangers such as those based on coal, brown coal and peat, as well as inorganic exchangers such as those based on greensand, kaolinite and zeolite. According to these methods, the radioactive substance is eventually concentrated in either the sludge (the precipitation method) or regenerated liquid wastes (the ion-exchange method). In the treatment of short lived radioactive liquid wastes, the concentrated liquid may be discarded after storage for a certain period so long as purified water has become available. However, in the treatment of long lived radioactive liquid wastes, a secondary treatment is necessary for immobilizing the concentrated radioactive substance so as to avoid subsequent diffusion. After the sludge or regenerated liquid waste is dewatered and its volume is reduced by the secondary treatment, it is converted into a solid form. The volume reduction rates achieved are &lt;1, 4, 5-10, 10-15, 25-35 and 2-4 respectively by secondary treatment such as precipitation, filtration with such as sand, filtration under pressure, vacuum filtration, centrifuging and autoclaving. The ideal material for collecting radionuclides present in radioactive liquid wastes should satisfy, among other thing, the requirements for rapidity and high efficiency. Furthermore, the material that has collected nuclides should desirably be disposed of by incineration. However, no conventional collecting materials satisfy all of these requirements.
While developing a material for collecting radionuclides present in radioactive liquid wastes discharged from nuclear power plants and other facilities using radioisotopes, the present inventors looked to the concept involved in collecting radionuclides present in extremely low levels in the seawater and rivers. Materials suitable for collecting radionuclides present in extremely low levels in the seawater and rivers require a high degree of simplicity in handling, rapidity and selectivity. For this purpose, materials such as ion-exchange resins that rely on equilibria between adsorption and desorption are not suitable; more preferred are inorganic adsorbents that are carried on suitable supports and which adsorb extremely low levels of nuclides by coprecipitation, surface adhesion or internal adsorption. The present inventors applied this idea to the development of a material for collecting radionuclides present in radioactive liquid wastes discharged from nuclear power plants and other facilities using radioisotopes. The material developed by the present inventors comprises an inorganic adsorbent carried on a support that can be disposed of by incineration. By using this material, not only nuclides that are present in extremely low levels in the seawater and rivers but also those nuclides which are present in radioactive liquid wastes discharged from nuclear power plants and other facilities using radioisotopes can be collected readily, rapidly and selectively. If necessary, the material that has collected such nuclides may be disposed of by incineration. Therefore, the present invention provides the ideal material for collecting not only nuclides present in extremely low levels in the seawater and rivers but also those which are present in radioactive liquid wastes liquors.
An incidental advantage of the collecting material according to the present invention is that it may be used as a means for concentration that is performed as a preliminary treatment for analyzing trace heavy metal elements, particularly those which are harmful and may cause pollution, or for recovery of precious metals.
The collecting material according to the present invention comprises an acrylic fiber element which fixedly supports a ferrocyanide compound of the formula K.sub.2 M.sup.II [FeCo(CN).sub.6 ] (wherein M.sup.II represents a divalent metal such as cobalt, zinc, zirconium or nickel) and/or manganese dioxide (MnO.sub.2). This material has been proved to have the capability of collecting various nuclides such as .sup.54 Mn, .sup.65 Zn, .sup.144 Ce, .sup.137 Cs, .sup.59 Fe and .sup.60 Co in an efficiency almost equal to 100%.
In this specification, the collective term "radionuclides" will be used to denote not only radionuclides present in extremely low levels in the seawater and rivers but also those radionuclides which are present in radioactive liquid wastes produced in nuclear power plants and other facilities using radioisotopes.
In the prior art, the attachment of adsorbents to carriers has been described by various expressions depending upon the nature and properties of the adsorbents and carriers, as well as upon subtle differences with respect to the method of attachment; among the expressions conventionally used are load, coat, impregnate, contain, bind, incorporate, composite, mix, treat, couple, retain, combine, support, immobilize and modify. It should be understood that the term "support" as used therein covers and is synonymous with all of these expressions.